In the Laboratory
A Diode-Laser-Based Automated Timing Interface for Rapid Measurement of Liquid Viscosity
W
R. Craig Urian and Lutfur R. Khundkar* Department of Chemistry, Northeastern University, Boston, MA 02115
As professionals, analytical or physical chemists are often involved in the design of laboratory hardware or in developing computer interfaces. This element of practical training is often missing from undergraduate laboratory exercises. Although programming a laboratory hardware interface is a much easier task today with the availability of commercial programs (e.g., LabView), the task of choosing an experiment that even minimally illustrates elements of hardware design poses a significant challenge. Our search for such an exercise was constrained by two requirements: it should have some practical consequence of contemporary interest, while being simple enough for an undergraduate to design and construct in two or three laboratory periods fragmented by the usual demands of regular course-work. One project we have recently incorporated in the final quarter of the undergraduate physical chemistry laboratory course is the design of an automated timing interface for measuring liquid viscosities. Viscometry of polymer solutions is an important method for characterizing polymers (1, 2) and is widely used in many laboratories involved in the design, testing, and manufacturing of polymers for various purposes (e.g., pharmaceuticals, paints, emollients, lubricants). Our implementation is aimed at providing students hands-on experience with solid-state electronics using a format that challenges them to think independently. Only the goal of the project is defined at the outset, and a team of two students is asked to develop a method of solution. At the end of one week, the instructor provides feedback on the proposed solution and offers additional guidance. Once a general design has been identified, the student team is asked to make a hierarchical list of tasks and provided with data sheets of relevant digital circuits, a breadboard, jumpers, and simple instructions on how to use them. In the final step, they assemble the instrument and demonstrate its functionality. Students are uniformly very enthusiastic about the project, often rating it as their favorite exercise, citing the introduction to elementary digital circuits as the most interesting aspect. Apparatus The principle of laminar flow utilized in many commercial viscometers (Ostwald, Ubbehold) allows us to correlate viscosities with total flow time. The interface relies on the fact that the meniscus of most transparent liquids in glass is curved and will refract light. A collimated narrow beam of light passes through a measuring tube (MT) containing the *Corresponding author. W Supplementary materials for this article are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/1998/ Sep/abs1135.html .
liquid whose viscosity is to be determined. The light beam, retroreflected through the same tube using a right-angle prism, illuminates a photodiode (detector). A single beam is thereby used to define two reference “lines” in the tube. When the meniscus intercepts either line, the light beam is refracted and will not reach the detector for a short period. This generates two high-to-low transitions, one when each beam is intercepted, which can be used as trigger signals. A simple logic circuit gates a clock signal for the period between these two trigger signals. Finally, a resettable pulse counter is used to count the number of clock pulses that pass through the logic gate. The most expensive part of this apparatus is the diode laser. A relatively inexpensive one ($250) and low-output power model may be obtained from Thor Labs, NJ.1 A simple box constructed out of black cardboard can be used to contain stray radiation. A softly focusing lens maintains beam widths less than 200 µm within the tube through which the liquid flows. A Si PIN diode (PD, < $10) is amplified (Analog Devices, AD524) tenfold and routed through a TTL Schmitt trigger inverter ("14) to generate clean timing pulses. This output is routed to a JK-type flip-flop ("73). A cheap crystal oscillator (2.000 MHz, 50 ppm) and Schmitt trigger combination is used to generate the master clock signal. A digital divider circuit consisting of a series of cascaded "390 is used to generate a variable signal clock frequency (typically 2 kHz). A TTL “AND” gate is used to count clock pulses arriving between two pulses from the JK flip-flop (start and stop).2 The counting device in our implementation is a standalone, resettable, TTL-compatible totalizing counter ($50, Allied Electronics). Discussion We estimate that the intrinsic error in timing is proportional to the ratio of the separation between the light beams and the beam waist. Our measured precision for repeated measurements with the same MT is ~0.5% for a total flow time of ~1 s, which reflects additional sources of error—for example, mechanical vibrations, local pressure or temperature fluctuations. These would have to be eliminated if higher precision were desired. The interface itself can be easily adapted to work with a standard Ostwald viscometer, using two detectors. The electronics would need slight modification to accommodate the second channel. Although the speed of data collection will depend on the viscometer design, the interface will eliminate the tedium and imprecision of manual timing. The high precision of timing (
